Bounce SuggestionsContext.SetSuggestions to UI thread

SetSuggestions has two callers: prepareConfirmationPanel calls it directly on
the UI thread, while editors.promptEditor and
SuggestionsContext.RefreshSuggestions call it via AsyncHandler, which runs the
result closure on a worker goroutine. The worker path currently relies on
HandleRender's self.c.Render() to flush the view update. Wrap the body in
OnUIThread so the worker path stays correct when Render() is removed; for the
UI-thread caller the extra bounce is harmless.